The invention relates to a circuit arrangement in a color television encoder, comprising a first oscillator for generating a chrominance subcarrier signal, a second oscillator for generating a signal having the line frequency or a multiple thereof, signals of field frequency being derived from this signal, and a mixer circuit for coupling the frequency of the chrominance subcarrier and the line frequency.
In such a circuit arrangement the frequencies of the two signals generated by the two oscillators are coupled to one another so that one frequency is higher than the other frequency by a given factor, this factor having a prescribed value in the relevant color television standard. The chrominance subcarrier has also a fixed phase relationship during consecutive line periods. For the NTSC standard the above-mentioned factor is chosen such that the phase of the unmodulated chrominance subcarrier is subjected to a 180.degree. variation between a given instant in a line period and the corresponding instant in the next line period. However, there is no prescribed phase relationship between the chrominance subcarrier and the signal of the line frequency.
An article in the publication "E.B.U. Review-Technical No. 172, December 1978, pages 265-281 describes a number of distrubances which may occur when color television pictures, in accordance with the PAL standard, are recorded, which disturbances are caused by the fact that the PAL video signal has an eight-field structure, that is to say with a repetition rate of 6.25 Hz (in Europe). The article advocates the desirability of eliminating these disturbances by means of a stable phase relationship between the two signals, the phase being defined as the phase of the -E.sub.u ' component of the color burst in the center of the leading edge of the preceding line synchronizing pulse, this component being obtained by means of extrapolation, and E.sub.u ', indicating, as known, one of the quardrature modulation directions. While the above-mentioned article indeed only relates to the European PAL standard, it will, however, be obvious that similar disturbances may occur when colour television pictures in accordance with other variants of this standard, and also in the case of the NTSC standard, are recorded.
Such a phase relationship can be obtained by means of adjustable phase shifting networks, which networks must be so stable that no shift can occur anymore between the burst and the synchronizing signal when they arrive at the input of the pick-up device. It is, however, difficult to realize such networks, whereas the phase of the two signals must be very stable with respect to temperature effects. In addition, the phase is not unambiguously determined on switch-on.